Instrument for measurement of optical characteristics of water

ABSTRACT

Apparatus for determining underwater optical characteristics is disclosed. One is the absorption coefficient (a) using a small cosine detector of area A at a distance R from a small (quasipoint) source based on the equation:   WHERE PD is the radiant power incident upon the detector. By using spectral filters in the detectors, coefficients a and b can be measured as functions of optical wavelength. The coefficient of total scattering s may be determined by measuring the coefficient of attenuation Alpha and subtracting from it the absorption coefficient a.   WHERE Pd is the radiant power incident upon the detector and Po is the radiant output of the source. Another is the backscattering coefficient (b) using either the same detector rotated 180* or a second detector based on the equation: PBZ Poe aZb dZ where PBZ is the radiant power of all backscattered light from a volume element dZ thick at radius Z, and a is the absorption coefficient. Allowance is made in determining b for the fact that a small cosine detector is employed of area A in accordance with the equation

United States Patent Glenn P. Sorenson Woodside, Calif. [21] ApplNo.790,280

[72] Inventor [22] Filed Jan. 10, 1969 [45] Patented Nov. 9, 1971 {73]Assignee Stanford Research Institute Menlo Park, Calif.

[54] INSTRUMENT FOR MEASUREMENT OF OPTICAL CHARACTERISTICS OF WATER 6Claims, 6 Drawing Figs.

52 U.S.Cl 250/218,

250/435 D, 356/103, 356/201, 356/206 51 lnt.Cl ..G0ln2l/26 50 FieldofSearch 250/218,

[56] References Cited UNITED STATES PATENTS 3,416,865 12/1968 Townsend356/206 OTHER REFERENCES The Determination of Atmospheric Transmissivityby Backscatter from a Pulsed Light Separated System, Armed Serviceslnformation Agency, 243 930, Dec 21, 1967.

Horman, Measurement of Atmospheric Transmissivity using BackscatteredLight from a Pulsed Light," J. of the Optical Society, Vol. 5l,No.6,.lune, 1961, p. 681- 690.

Tyler, J. E and Richardson, Nephelometer for the Measurement of VolumeScattering Function In Situ, .l. of the Optical Society, Vol. 48, No. 5,May, 1958, pp. 354- 357.

Tyler, 1. E., Measurement of the Scattering Properties of l-lydrosols,".l. of the Optical Society of America, Vol. 51. No. 11,Nov.,1961,pp.1289-1293.

Primary Examiner-James W. Lawrence Assirtanl Examiner-P. C. NelmsAttorneys Urban Faubion and Lindenberg & Freilich ABSTRACT: Apparatusfor determining underwater optical characteristics is disclosed. One isthe absorption coefficient (a) using a small cosine detector of area Aat a distance R from a small (quasi point) source based on the equation:

where P,, is the radiant power incident upon the detector. By usingspectral filters in the detectors, coefticients a and b can be measuredas functions of optical wavelength The coefficient of total scattering 5may be determined by measuring the coefficient of attenuation a andsubtracting from it the absorption coefficient a.

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i I9 I8 H-VDC LAMP WDETECTOR POWER SUPPLY 5/ ()l CONTROL foLTMETER GLENNP. SORENSON BY ATTORNEYS I PAIENTEUunv 91971 SHEET 2 UF 2 PULSEGENERATOR LOW | PASS LOGARITHMIC RATIO MODU LE PEAK DETECTING VOLTMETERGLENN P. SORENSON ATTORNEYS INSTRUMENT FOR MEASUREMENT OF OPTICALCHARACTERISTICS OF WATER BACKGROUND OF THE INVENTION This inventionrelates to apparatus for determining underwater optical characteristics,and in particular to apparatus and method for determining absorption,total scattering and backscattering characteristics of particular bodiesof water at desired depths.

Recent activity in oceanography has given rise to a need for a widevariety of instruments, many having optical systems for use at suchdepths that little or no light would be available from the surface. Tosatisfy that need, new instruments must be developed, includinginstruments with which to predict underwater visibility from measuredcharacteristics of the local water mass, and to determine actual opticalcharacteristics of the water during underwater tests of instrumentshaving optical systems undergoing evaluation.

While the primary interest is in determining optical characteristics ofdeep ocean water under conditions of artificial illumination, themeasurement of such characteristics of fresh or shallow water areequally useful. It is also useful to measure the optical characteristicsof turbid water, both deep and shallow, in order to predict visibilityunder specific turbidity conditions. Accordingly, although referencewill be made hereinafter to measuring characteristics of sea water underconditions of artificial illumination, it should be understood that theinstrument concepts are equally useful in measuring the opticalcharacteristics of shallow water (both fresh and ocean water) and inconnection with visibility under ambient light conditions. Such conceptsare also useful in marine biology (measurements of absorption andscattering), physical oceanography (measurements of fine structure ofwater turbidity), commercial oceanography (photography, search) andcommercial applications.

The characteristics of sea water which must be measured, or accuratelyestimated from other measurements, in order to predict or compareunderwater visibility under conditions of artificial illumination are:the attenuation coefficient a; the volume absorption coefficient a; thetotal scattering coefficient s; the backscattering coefficient b; andthe distribution function of small forward scattering angles, i.e., thesingle scattering angular flux distribution in the region betweenmaximum and about half-maximum intensity (from zero to about 1?).

lt is assumed that the distribution function of small forward scatteringangles is sufficiently similar in most natural ocean waters to generallyalleviate the need for measuring that function in oceanographic studies.It may sometimes also be assumed that the ratio bzs is sufficientlysimilar in most natural ocean waters to permit 5 to be estimated from b.Besides the attenuation coefficient, it is then necessary to measureonly the backscattering coefficient b and the absorption coefficient a.In any case, it is useful to measure at least those two characteristicsof water because absorption and scattering is due to particulate organicand inorganic materials in the water which can be expected to varywidely. That is unlike the atmosphere where absorption is normallynegligible. On the other hand, since the coefficient of attenuation a isthe sum of absorption and scattering coefficients a+s, if thecoefficients a and a can be measured, then the coefficient s can bedetermined.

The scattering of light by the water is similar to the scattering oflight by the atmosphere in that scattering in each media is verynonisotropic. However, the scattering of light in water is much moreintense than in the atmosphere, with average distances betweenscattering events of the order of meters rather than kilometers. Thus,optical characteristics of sea water that affect visibility includescattering as well as absorption properties. Both properties limitvisibility by removing illumination between the light source and thetarget, and imagebearing light between the target and the sensor.Backscattering can also affect visibility by reducing contrast, a

phenomenon well known to all those who have attempted to drive anautomobile through the fog with headlights.

Attenuation of light between the source and the target can be closelyapproximated through the use of the volume absorption coefficient a. Forexample, the ratio of underwater illumination I from a point source atdistance R to output radiant power P, is approximately equal to theratio e"":41rR where I is power per unit area and the absorptioncoefficient a is the reciprocal of the average distance each photontraverses before being absorbed. The term power" as used herein to referto radiant power or illuminating power is the standard measure of powergiven by the product of photons per second and the energy per photon.

While absorption and scattering of light are factors which interact in acomplex manner, it is assumed that the details of the interaction can bespecified in a mathematical model. In terms of instrumentation, the taskthen is to: measure the attenuation coefficient a; measure theabsorption coefficient a and the backscattering coefficient b; anddetermine from other measurements the total scattering coefficient s. itis then possible to predict visibility or response of a sensor of lighttransmitted from a source to a target.

In predicting visibility in sea water, two phenomena should beconsidered. The first is the eflect of wavelength on attenuation. Exceptin coastal waters where appreciable quantities of yellow-colored organicmaterial from decaying shore life are dissolved, sea water tends toabsorb wavelengths outside a 0.48i0.05 ,a region much more quickly thanlight inside that region. Accordingly, sea water is regarded as having awindow for light in the blue-green region of the spectrum. Watercharacteristics are, therefore, customarily measured only within theblue-green region of the spectrum.

The second phenomenon is that scattered light that reaches the targetserves to illuminate the target just as effectively as unscatteredlight. However, since scattering r is mostly at narrow forward angles,the average path length actually traversed by light from the source tothe target is only slightly greater than the straight line path length,and so the actual attenuation experienced due to absorption is only veryslightly greater than e at ranges of several scattering mean freepaths'or less. Accordingly, the absorption coefficient a is anappropriate measure of attenuation of scattered light, i.e., actualattenuation is very closely approximated by e even at ranges of severalscattering mean free paths. This makes prediction of attenuation bymeasurement of only absorption possible. By then measuring backscatter,contrast can also be predicted.

SUMMARY OF THE INVENTlON In accordance with the present invention,characteristics of a body of water which affect visibility aredetermined by measuring the coefficient of attenuation a, thecoefficient of absorption a, the backscattering coefficient b anddetermining the coefficient of total scattering s by subtracting thecoeffcient a from the coefficient a, or by estimating it from themeasurement of backscattering b. The coefficient a is measured by anarrow-angle light transmitter and receiver (preferably with very narrowtransmitter and receiver beamwidth half-angles). The radiant power inputto the detector is then due to light unabsorbed and unscattered (exceptthrough very narrow forward angles) over a predetermined path length.The absorption coefficient a is measured directly by a sensor facing thesource of light after calibration. The backscattering coefficient isdetermined by measuring the illuminating power detected by a sensorfacing away from the direction of the light source. and from thatmeasurement, calculating the coefficient b. Since the apparatus formeasuring the coefficients a and b are very similar, the only differencebeing the direction in which the detector is facing, the samecalibration data are employed for determining the coefficient b as formeasuring the coefficient a in accordance with Eq. 1 l set forthhereinafter.

An alternative method of determining the total scattering coefficient sis to determine the backscattering coefficient b and estimate thescattering coefficient from the coefficient 1;. That is valid for bodiesof water in which the ratio of bzs is substantially constant, such asnonturbid sea water.

The apparatus for measuring the absorption coefficient :1 comprises alight detector facing a source of light at a predetermined distance R.Both the light source and the detector are small so that the lightsource approaches a pointsource, and the detector approaches a smallsegment of a sphere of radius R even though its face be flat. A flatdiffuser in front of the detector provides a cosine response to thedetector. Calibration is achieved by effectively changing the distance Rin homogeneous water from R to R or vice versa. The ratio of radiantpowers Pd,:Pd measured at the two known distances provides thecoefficient a in a self calibrating manner in accordance with Eq. (2)set forth hereinafter. That value of the coefficient a is then used aspart of the calibrating data required for determining the coefficient b.After calibration, the coefficient a may be found from a single radiantpower measurement Pd at a fixed distance R in accordance with Eq. (4)set forth hereinafter. The balance of the calibrating data required fordetermining the coefiicient b from radiant power measurements (with thedetector facing away from the source, instead of toward it as fordetermining the coefficient a is attained pursuant to Eq. l) set forthhereinafter.

Although a single detector mounted on a frame may be employed for allmeasurements, three detectors are preferred, two at distances R and Rfor calibrating and determining the absorption coefficient a and a thirdone mounted at a distance R for determining the backscatteringcoefficient b. Each detector is used with the same light source, but sospaced as to not interfere with transmission of light to otherdetectors. A flat diffuser is placed near each detector with its faceparallel to the face of the detector in order to provide a cosineresponse to the detector. Each detector may be provided with a filter tolimit its response to some specific portion of the spectrum, forexample, to the blue-green region of the spectrum. Other filters may beprovided as desired, such as neutral density filters.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionwill best be understood from the following description when read inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS I FIG. I shows a hemisphere with asegment thereof illuminated by a point source to illustrate the basis ofthe present invention.

FIG. 2 shows variation of detected radiant power with range as would bemeasured by a hypothetical meter of attenuation a with a detector havinga variable acceptance angle and a very large aperture to illustrate theprinciple of the present invention.

FIG. 3 shows diagrammatically a sideview of one exemplary form ofmeasuring apparatus according to the invention.

FIG. 4 shows a block diagram of the electronic system for the presentinvention illustrated in FIG. 3.

FIG. 5 illustrates diagrammatically another exemplary form of thepresent invention.

FIG. 6 shows a block diagram of an electronic system for the exemplaryform of the invention illustrated in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS PFPO 4m where; P =illuminatingoutput power of source within the bandwidth of the detector;

A=area of detector;

at absorption coefficient. The area A is ideally a segment 10 of aspherical surface 11 surrounding a point source of light 12, as shown inFIG. 1, but may be a flat plate if sufficiently small to approximate thesegment 10. For simplicity of the drawing, only half of the sphericalsurface is shown. Thus, the system can first be visualized as aspherical irradiance detector with a point source at the center, thenvisualized with the detector covering only a small segment of thesphere, and finally realized as a flat detector having an area A muchsmaller than in-R The ratio of illuminating power incident on the flatdetector (P,,) to the total irradiance power reaching the entireimaginary spherical surface (P e is substantially equal to the ratioAz4'n'R Since I the distance R and area A are known, and P can bemeasured as the power output of the source, the coefficient ofabsorption a can be determined by measuring the detected power P,,.

The principle of the present invention may be better appreciated fromthe diagram of FIG. 2 which shows the theoretical variation of powerfrom a light source with distance R. FIG. 2 is a logarithmic plot ofdetector power (in P measured by a hypothetical meter of attenuation.Such an attenuation meter may consist of a stable source of light, suchas a tungsten bulb with a constant current through it, illuminating adetector with a large active area. A mask with a variable opening isplaced in front of the detector to vary the acceptance angle of thedetector. As the acceptance angle 9 closely approaches zero, themeasured power would be as shown by the lower curve labeled e'a". As 6is increased, the measured power at a given distance R would alsoincrease. When 9 approaches the measured power will closely approach eThis is because all of the unabsorbed, forwardscattered photons aremeasured. The difierence in slope between the curve labeled 9=w and thecurve labeled e represents the fact that scattering causes some flux totravel a distance greater than R. This scattered flux has a higherprobability of being absorbed, thereby decreasing the measured power eto below the level 9=1r if no scattering takes place. The difference,however, is very small. Independent research has verified this closerelationship between the curves e and 6=vr. Accordingly, the basis forthe present invention is that the absorption coefficient a may bedetermined from Eq. I) if radiant power is measured with a detectorhaving an acceptance angle approaching 180". In otherwords, the radiantpower detected (P,,) from the source will decrease directly with thesquare of distance R and further decrease exponentially with thedistance R for a given absorption coefficient a. Therefore, knowing P Aand R permits determining the absorption coefficient a from a detectorpower measure ment P One simple form of measuring apparatus for theabsorption coefficient a is shown in FIG. 3. It comprises a supportingframe 15 having a rigid upright arm 16 and a pivoted upright arm 17. Onearm supports a cosine detector 18 and the other supports a light source19.

A cosine detector is a light-measuring instrument which has alight-collecting surface known as a cosine (or lambert) collector thatwill provide measurements which stay constant as the collecting surfaceis turned in all possible ways to accept a beam of fixed amount and typeof radiant energy. In other words, the cosine detector reading isindependent of the angle of incidence of a beam of radiant energy on itscollecting surface.

A cosine collector" usually consists of a flat diffusing (translucent)material typically opal glass. Thus, a lightmeasuring instrument whichincorporates a cosine collector is commonly called a cosine detector"and typically consists of a detector (photomultiplier, photodiode. andthe like) positioned closely behind a piece of flat diffusing material,the only light reaching the detector passes through that material.

In the exemplary form of the present invention illustrated in FIG. 3,the cosine detector 18 comprises a photomultiplier 20 having itscollecting surface 21 positioned closely behind a piece of opal glass22. The photomultiplier tube is mounted within a water and lighttighthousing represented schematically by a dotted line 23. Connection to thephotomultiplier is through a watertight cable 24.

Another pivoted arm 25 is provided to facilitate varying the distance Rfrom one predetermined value R, to another R By making both pivoted arms17 and 25 the same length, and pivotally connecting them to a base 26for the light source 19 the same distance apart as to the frame 15, theorientation of the light source to the detector is maintained constantas the distance is varied. Rigid pins 27 and 28 pass through the arms 17and 25 and into the path of the frame so that, as the arms 17 and arepivoted, the pins 27 and 28 limit the travel of the light source 19 totwo extreme positions which define the distances R, and R A second rigidarm 29 on the frame 15 anchors a spring 30 connected to the pivotal arm17 to hold it in each of two extreme positions as shown.

The light source 19 may be, for example, a 100 watt, 12 volt,quartz-iodine light bulb within a watertight Pyrex en velope. A Wrattenfilter is placed over the flat diffuser (opal glass) 22 in the window ofthe housing 23 to limit response to the blue-green region of thespectrum. Different neutral density filters may also be selectivelyplaced over the flat diffuser 22. The window of the housing 23 coveredby the flat diffuser 22 is made sufficiently large for the acceptanceangle of the cosine detector 18 to approach 180.

The detector housing 23 is adapted to rotate 180 to measurebackscattered light as will be described more fully hereinafter. Foreither type of measurement, the electronic system of F 10 4 is used. Itcomprises a storage battery connected in series with the filament of thelight source 19 and a current monitor and control module 41. The lattermay consist of a digital voltmeter for measuring current through thefilament of the light source 19 and one or more potentiometers foradjusting the current. The detector 18 is provided with a high voltage(1,250 VDC) power supply 42 and a digital voltmeter 43 which measuresthe voltage across a load resistor 44 in series with the detector 18 Theoutput of the digital voltmeter 43 is then proportional to the radiantpower P, detected.

An important characteristic of the present invention is that it isself-calibrating. By simply changing the distance-R, to the distance Rin homogeneous water, the device is calibrated. in other words, thepresent invention only requires that the distance be varied from onepredetermined value R, to another known value R in order that theabsorption coefficient be determinable from the following equation:

Thus, the ratio Pd,:Pd of radiant powers detected at two known distancesprovides a measurement of the coefficient a. This method is independentof the output power P, of the source 19, and permits easy and frequentrecalibration. However, photometric calibration may be undertaken andonce calibrated, by either method, the distance may be kept constant andthe coefficient a may be found from a single detector power measurementP,,, assuming P remains constant, from the following equation:

lnC1nP where C is a constant obtained by a calibration operation basedon Eq. l and has the following value:

In other words, having found the coefficient of absorption a inaccordance with Eq (2) the value ofC may be found from Eq.

( 1) by using that value of the coefficient and, for example, the powermeasurement P, at distance R, in accordance with the following equation:

The advantage of usingihTs technique is, for example, being able toobtain the absorption coefficient a at different depths with just onemeasurement of radiant power Pd at each level, instead of two as for theself calibrating technique of Eq. (2).

Errors in the values of the absorption coefficient determined by thismethod and apparatus can be caused by variations in power output Perrors in measurement of distances R, and R and errors in measurement ofthe voltage across the load resistor 44 (Le, the power of irradiationdetected). Detector noise is too low to be a source of error with a wattquartz-iodine light bulb for the source 19 and a photomultiplier tubefor the detector 18. Error due to ambient light can be eliminated byoperation in the dark, or by measuring the difference in the lightdetected with the source on and off, or by using a pulsed light sourceof high intensity and matched amplifier circuits following the detectoras will be described with reference to FIG. 5 and 6. Thus, only inherenterrors are of any real consequence.

Considering all possible errors, including errors due to the lightsource 19 being somewhat nonisotropic and not a point source, overallaccuracy achievable in the measurement of the absorption coefiicient ais believed to be within 10.01 m.". This, of course, is on theassumption that scattering of light will not cause the photon path fromthe source to the detector to vary significantly. This is true in allwaters except those within the surf zone or within the region ofsuspended sediment near a disturbed bottom, for in all other parts ofthe ocean, the average nonabsorbed photon does not travel significantlymore than the distance between the source 19 and the detector 18. Thisis so because scattering is predominantly at narrow forward angles, andbecause the scattering mean free paths are long compared to likelydimensions of R and R the longest ofwhich would not exceed 1 meter. Inpractice R, may be 0.65 m. and R, less than 1 meter. Thus, in waterwhere the forward scattering coefficient s does not exceed 0.2 rn., Eq.(1) on which the present invention is based may be used in spite ofsmall inherent errors of even a few percent in values of the absorptioncoefficient as that would not be significant and would, in fact, be lessthan errors in measurement.

For more rigorous measurements. an error analysis may be made andcorrections applied, and for greater precision in establishing R, and Rtwo cosine detectors 50 and 51 may be permanently mounted on a frame 52at fixed distances R, and R from a source 53 as shown in FIG. 5. Thesource 53 is so rigidly mounted on the frame 52 as to be betweendetectors 50 and 51, but not in a straight line because if the twodetectors were to face each other on astraight line the face of eachwould reflect spectrally into the other and induce errors.

To measure the backscattering coefiicient b, the irradiance detector 18of FIG. 3, employed to measure the absorption coefficient a inaccordance with Eq. (2) or (4), is rotated so that it faces away fromthe light source. Light backscattered from a concentric sphere ofgreater radius Z is then detected except from that volume of the body ofwater in the shadow of the detector itself which is not significantbecause of the small solid angle involved. The power P of light thatreaches the entire surface of the larger sphere at distance 2 from thesource will be about The backscattered power P from a volume element dZthick at distance Z will be:

P =P e b (12 (7) where the backscattering coefficient [1 representsscattering at angles of more than 90. Assuming the detector is acomplete s here and is transparent to light radiating out from thesource, but not to light backscattered from the source, but not to lightbackscattered from the larger sphere, the power at the detector P, wouldbe given by the following equation:

Since only a small flat detector is employed of area A, the power at thedetector P is given by the following equation:

This approximation assumes that all photons backscattered to thedetector travel a distance of 2Z-R. Increases in path length due tomultiple scattering can be ignored just as in making absorptioncoefficient measurements.

A convenient way of calibrating the instrument for backscattermeasurements is to use measured values of R and V and the determinedvalue of the absorption coefficient a in the following equation:

P 'oA=41rR l ,e"'* 10) where V 1 is a voltage measurement across theload resistor 44 instead of a detected power measurement, and P,, is inunits of volts; otherwise, Eq. (10) is the same as Eq. (1), butrearranged. The backscattering coefficient may then be calculated fromthe following equation:

where V is the output voltage of the detector made with the same loadresistor 44 as in measuring V The quantity P A is a constant for anygiven set ofconditions relating to source, detector, load resistor andvoltage meter.

Measurements of the backscattering coefficient are generally subject tothe same errors as measurements of the absorption coefficient a whichare not more than a few percent for water having total scatteringcoefficient s of not more than 0.2 m.", as noted hereinbefore.Additionally, backseattering (scattering within the back hemisphere) issomewhat nonisotropic and error in estimation of the distributionfunction within the back hemisphere will cause an additional inherenterror. However, the distribution function appears to be quite similaramong different waters. When errors of all sources are considered,including occulting of the sector near 180 by the detector, overallaccuracy achievable in the measurement of the backscattering coefficientb is believed to be within :10 percent. For more rigorous measurements,an error analysis may be made and corrections applied.

Although a single detector can be employed for determining bothabsorption and backscattering coefficients by so mounting the detectorthat it can be rotated away from the source, as noted hereinbefore withreference to FIG. 3, it is preferred to have a separate detector 54permanently mounted on a supporting frame 52 facing away from the source53 as shown in FIG. 5. Thus, a three-detector system is preferred forabsorption and backscatter coefiicient measurements. Once thosemeasurements have been made, the total scattering coefficient s may bedetermined.

The total scattering coefficient (the reciprocal of the scattering meanfree path) is an important measure of visibility range. In general, thecoefficients a and s must be known separately in order to determine thevisibility-limiting characteristics of water. Since the attenuation ofimage-bearing light between the target and the sensor expressed by thecoefficient a can be measured, and that coefficient is equal to the sumof the coefficients a ands, measurement of the coefficients a anduyields the forward scattering coefficient s. Alternatively, in mostnatural (nonlurbid) ocean waters .r can be estimated from b once theratio b:r has been determined since the ratio bzr is sufficientlyconstant at different depths and locations of the same body of oceanwater.

Although the power of light detected by each of the cosine detectors inthe arrangement of F IG. 5 could be measured by the arrangement of FIG.4, it is preferred to use a pulsed light source, such as on Xenon lamp,while making measurements of the absorption coefficient a, therebyreducing the effect of photocurrent resulting from background light,i.e., ambient light from the sky, sun, etc. Each light pulse waveenvelope is ideally rectangular and emanates from the source 53 whichapproximates a point source. However, in practice the wave envelope ofthe light pulse may be of the form customarily provided by an Xenon lampfor photographic use, which is with a rapid rise to peak intensityfollowed by a less rapid fall.

The signal from 10 given detector is passed through a lowpass RC circuithaving a time constant T, less than the pulse duration T of, forexample, 0.3 ms. in duration and through a high-pass filterimpedance'matching amplifier with a matching time constant 1 at least 10times the pulse-to-pulse time interval. If r =0.25 T and the effectivebandwidth Af of the electronic band-pass filter is l 21,), then Af=2/T.It may be shown that these time constants will result in a 90 percentfilter response and good pulse-to-pulse discrimination. The resultingsignal-to-noise ratio at the output of the amplifier is percent of thesignal-to-noise ratio at the input of the amplifier.

A pulse generator 55 is provided to trigger the light source 53 in amanner well known to those skilled in the art. An electronic flash unitmay be provided with a fixed flash rate, or a flash rate which can bevaried in the same manner as for photography. Alternatively, the pulsegenerator 55 may be adapted to trigger the Xenon light source 53 inresponse to a manually actuated switch to produce the desired lightpulse. in either case, the pulse generator 55 is preferably enclosedwithin the watertight housing for the Xenon lamp. For manual operation,a cable may be provided to trigger the pulse generator 55 from outsidethe body of water.

Fig. 6 shows a block diagram of an electronic system for thetransmission and conversion of detector signals to a voltage signal Eproportional to the absorption coefficient using a pulsed light source.Each of two low-pass filters 57 and 58 is provided with the timeconstant 1, and each of two impedance-matching amplifiers 59 and 60 isprovided with the time constant 1' The optimum value for load resistors61 and 62 is determined (by the inherent capacitance C of the detectorsand the light pulse width T) to be the reciprocal of ZCAF, where Af isthe electronic frequency cutoff equal to 2/1". The impedance-matchingamplifiers 59 and 60 may be, for example, emitter followers in asolid-state system. They do not alter the output signals V and V acrossthe load resistors 61 and 62, but increase the signal power fortransmission through 50 9. cables 63 and 64 to a log ratio module 65. inpractice, the filters, load resistors, and impedance-matching amplifierswould be included in the watertight housings of the detectors 50 and 51.

The use of a single detector as described with reference to FIGS. 3 and4 provides ease of calibration, but requires that the detector berepositioned to the precise positions for the distances R, and R Errorsdue to variations in the distances R and R, can be substantial when theyare assumed to be as measured preliminarily. For example, a 5-mm.variation in the difference R R will cause an error in measurement ofthe absorption coefficient a of 0.04 in. when R -R, is intended to be 50cm. That is significant because open-ocean values of the absorptioncoefficient are generally within the range of about 0.02 m. to about 0.1m". Use of multiple detectors 50 and 51, on the other hand, will assuremeasurement accuracy if the distances between the detectors and thelight source is rigid. When that condition of rigidity is substantiallysatisfied, the output signals V and V 2 from the amplifiers S7 and 58(which signals are proportional to the power of light incident on therespective detectors 50 and Sl) may be applied to a logarithmic ratiomodule 63 that will take the logarithm of the ratio V :V, and providethe voltage signal E, directly proportional to the absorptioncoefficient a. This logarithmic-ratio transfer function makes real-timereadout through a peak-detecting voltmeter 66 possible.

Another advantage of using two light detectors illuminatedsimultaneously by a single light source is that current through thelight source need not be monitored to maintain the output power of lightconstant. In other words, the coefficient a can be measured independentof the source output power.

In order to utilize the most sensitive voltage input range of thecircuit module 65, the two voltage signals V, and V must be of about thesame level. To accomplish that, a neutral den sity filter 67 is placedover the detector 50, thereby offsetting the signal level of thedetector 50 due to its position closer to the light source 53. Otherwisethe closer detector would receive more light (if both are of the samearea A) because the solid angle of the closer one equal to A/R, isgreater than the solid angle A/Rf. Therefore, to exactly compensate, thefilter 67 must have a transmissivity of When the filter 67 is in place,any difference in voltage signals V, and V is due only to the absorptionof light in the water path of length R,-R,. Another way the outputvoltages from the two detectors 50 and 51 can be effectively equalizedis by using variable load resistors 61 and 62 or variable gainamplifiers Calibration may be accomplished in air (where a=0), oralternatively a second light source may be positioned equidistant fromthe two detectors 50 and 51 to provide in situ calibration by equalizingthe levels of the output signals V, and V Since system sensitivityincreases linearly with increasing path difference (R R,), an actualinstrument could theoretically be made with as much sensitivity asdesired by simply increasing the path difference. However, the length ofthe instrument must be limited due to other considerations, such asrigidity in the distances R, and R and light source power.

Rigidity is provided in a simple way by the frame 52 illustrated in FIG.5. If the third detector 54 is not required for backscatter coefficientmeasurements, or one of the detectors 50 and 51 is adapted to be rotated180 for use in such measurements, the frame 52 may consist of a simpletriangle with a ring 68 at the apex. For high strength-to-weight ratio,alu minum is the preferred material for the frame 52, When anodizedblack, it will reflect very little light from the source 53 to thedetectors. A for each side of the frame 52 is an extruded square tube 10feet long with Vii-inch walls. Greater length will increase sensitivityand decrease error associated with the logarithmic ratio module 63, butmay result in excessive flexure error due 59 and 60 to compensate forthe R differences. 30

preferred shape and maximum length to G" forces (acceleration) caused byheaving of the ship from which the instrument is suspended by a tautcable 69 attached to the ring 68, and drag forces due to relative motionof the instrument and the water. In each case, the force will actparallel to the cable 69.

If the third detector 54 is provided as shown in FIG. 5, it is providedwith a low-pass filter 70, load resistor 71 and highpassimpedance-matching amplifier 72 in order to couple the detector 54 toinstrumentation on deck through a cable 73 in a manner similar to theway the detectors 50 and 51 are coupled to instrumentation. Theamplifier 72 is connected to the peakdetecting voltmeter 66 via asuitable amplifier 74 by a switch 75 while making backscatteringcoefficient measurements.

Although exemplary embodiments have been specifically disclosed, itshould be understood that practice of the invention is not limited tothose embodiments. Modifications and variations falling within thespirit ofthe invention will occur to those skilled in the art.Therefore, it is not intended that the scope of the invention bedetermined by the disclosed exemplztn embodiments. but rather should bedetermined by the breadth ot" the appended claims.

Iclaim:

1. In apparatus having a small source of light, light detection meanscomprised of at least one light detector having its detection surface ofarea A facing said source at an arbitrary distance R for determining theabsorption coefficient a of a body of fluid on the basis of thefollowing equation:

whereP is the output power of said source, and P is radiant powerincident on said detection surface of said detection means, using thefollowing equation:

0 where P and P,,, are irradiation powers detected at two knowndistances R, and R the combination of means for spacing said detectionsurface of said detection means from said source at said distances, R,and R means for maintaining output power of said source constant, and

means for measuring radiant power incident on said detection surface ofsaid detection means at said two known distances R, and R while saidoutput power is maintained constant, said light detector of saiddetection means being a substantially flat cosine detector with saidlight detection surface area so small that it approaches the shape of asection of an imaginary sphere surrounding said source and passingthrough said light detection surface at each of said distances R, and R2. In apparatus having a small source of light, a light detector havingits detection surface facing away from said source for determining thebackscattering coefficient b of a body of fluid on the basis of thefollowing equation:

(I) e.=..f1( of where P is irradiation power detected of backscatteredlight incident on said detector,

means for spacing said detector with its detection surface at anarbitrary distance R from said source. means for maintaining outputpower of said source at a known constant P and means for measuring saidirradiation power Pp incident on said detector, said detector being asubstantially flat cosine detector with a detection surface having anarea so small that it approaches the shape of a section of an imaginarysphere of radius R surrounding said source. 3. Apparatus for measuringthe light absorption coefficient a of a body of fluid comprising a smallunshielded source of light and at least one cosine light detectordisposed with its light detection surface facing said source at apredetermined distance R to receive light therefrom transmitted directlythrough said body offluid, said light detector having a light detectionsurface of an area so small as compared to the surface area 4'n-R of animaginary sphere surrounding said source that said detection surface ofsaid detector approaches the configuration ot' a section of said sphere,said apparatus including means for maintaining the output power of saidsource constant, and means for measuring the power of light incident onsaid detector from said source.

4. Apparatus for measuring the light backscattcr coefficient 1 of a bodyof fluid comprising a small unshielded source of light and a cosinelight detector disposed with its light detection surface facing awayfrom said source at a predetermined distance R to receive backscatteredlight originating from said light source, said light detector having alight detection surface of an area so small as compared to the surfacearea 411'R of an imaginary sphere surrounding said source that saiddetection surface of said detector approaches the configuration of asection of said sphere, said apparatus including means for maintainingthe output power of said source constant, and means for measuring thepower of light incident on said detector from said source.

5. Apparatus for measuring the light absorption coefficient a of a bodyof fluid comprising a small light source and two flat light detectorsrigidly disposed at different distances about said source with theirlight detection surfaces facing said source for receiving lighttherefrom, each of said light detection surfaces of said two flat lightdetectors being of an area so small as compared to the surface area ofan imaginary sphere passing therethrough and centered around said sourcethat said light detection surface area approaches the configuration of asection of said sphere, each of said detectors being a cosine detectorfor producing a signal proportional to the power of light incident onits light detection surface, and means for producing an output signalproportional to said absorption coefficient a from the logarithm of theratio of signals from both of said light detectors.

6. Apparatus as defined in claim 5 for further measuring backscatteringcoefficient b of said body of fluid including a third light detectordisposed about said source in a position which will not occludetransmission of light from said source through said fluid to said otherdetectors, said third detector being oriented to receive light thereonfrom a direction directly away from said source and having asubstantially flat light detection surface of an area so small ascompared to the surface area of an imaginary sphere passing therethroughand centered around said source that said light detection surface areaapproaches the configuration of a section of said sphere.

1. In apparatus having a small source of light, light detection means comprised of at least one light detector having its detection surface of area A facing said source at an arbitrary distance R for determining the absorption coefficient a of a body of fluid on the basis of the following equation:
 2. In apparatus having a small source of light, a light detector having its detection surface facing away from said source for determining the backscattering coefficient b of a body of fluid on the basis of the following equation: where Po is the output power of said source and PBZ is power of light from said source backscattered from a volume element of said fluid dZ thick at radius Z from said source, and a is the absorption coefficient of said fluid, using the following equation:
 3. Apparatus for measuring the light absorption coefficient a of a body of fluid comprising a small unshielded source of light and at least one cosine light detector disposed with its light detection surface facing said source at a predetermined distance R to receive light therefrom transmitted directly through said body of fluid, said light detector having a light detection surface of an area so small as compared to the surface area 4 pi R2 of an imaginary sphere surrounding said source that said detection sUrface of said detector approaches the configuration of a section of said sphere, said apparatus including means for maintaining the output power of said source constant, and means for measuring the power of light incident on said detector from said source.
 4. Apparatus for measuring the light backscatter coefficient b of a body of fluid comprising a small unshielded source of light and a cosine light detector disposed with its light detection surface facing away from said source at a predetermined distance R to receive backscattered light originating from said light source, said light detector having a light detection surface of an area so small as compared to the surface area 4 pi R2 of an imaginary sphere surrounding said source that said detection surface of said detector approaches the configuration of a section of said sphere, said apparatus including means for maintaining the output power of said source constant, and means for measuring the power of light incident on said detector from said source.
 5. Apparatus for measuring the light absorption coefficient a of a body of fluid comprising a small light source and two flat light detectors rigidly disposed at different distances about said source with their light detection surfaces facing said source for receiving light therefrom, each of said light detection surfaces of said two flat light detectors being of an area so small as compared to the surface area of an imaginary sphere passing therethrough and centered around said source that said light detection surface area approaches the configuration of a section of said sphere, each of said detectors being a cosine detector for producing a signal proportional to the power of light incident on its light detection surface, and means for producing an output signal proportional to said absorption coefficient a from the logarithm of the ratio of signals from both of said light detectors.
 6. Apparatus as defined in claim 5 for further measuring backscattering coefficient b of said body of fluid including a third light detector disposed about said source in a position which will not occlude transmission of light from said source through said fluid to said other detectors, said third detector being oriented to receive light thereon from a direction directly away from said source and having a substantially flat light detection surface of an area so small as compared to the surface area of an imaginary sphere passing therethrough and centered around said source that said light detection surface area approaches the configuration of a section of said sphere. 